1. Field of the Invention
The present invention relates generally to piezoelectric resonators, and more particularly, to a piezoelectric resonator utilizing a second harmonic wave of the thickness-extensional vibration mode.
2. Description of the Prior Art
FIG. 7 is an exploded perspective view for explaining a conventional piezoelectric resonator utilizing a second harmonic wave of the thickness-extensional vibration mode. This piezoelectric resonator 50 has a piezoelectric substrate 53 formed by laminating a pair of ceramic green sheets 51 and 52 and sintering the same. A stripe-shaped inner electrode 54 extended in one direction is provided inside of the piezoelectric substrate 53. In addition, circular or polygonal outer electrodes 55 and 56 are formed on both outside major surfaces of the piezoelectric substrate 53 so as to be opposed to the inner electrode 54. The outer electrode 56 is illustrated in a frame represented by a broken line obtained by projecting the lower surface of the ceramic green sheet 52.
As shown in FIG. 7, in the conventional piezoelectric resonator 50 utilizing a second harmonic wave, the dimension of each of the outer electrodes 55 and 56 in the direction of the length of the inner electrode 54 is made equal to that in the direction of the width thereof.
FIG. 8 shows frequency-impedance characteristics of the conventional piezoelectric resonator 50 shown in FIG. 7. In FIG. 8, the ordinate represents an impedance Z and the abscissa represents a frequency f.
As shown in FIG. 8, the conventional piezoelectric resonator 50 has the disadvantage that a spurious mode S occurs on the side of a slightly higher frequency region from an antiresonance point A, thereby causing characteristics of the resonator to be degraded.